1. Field of the Invention
The invention pertains to the field of variable camshaft timing systems. More particularly, the invention pertains to a vent mechanism for venting air out of a variable camshaft timing system.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the angle between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more xe2x80x9cvane phasersxe2x80x9d on the engine camshaft (or camshafts, in a multiple-camshaft engine). In most cases, the phasers have a rotor with one or more vanes, mounted to the end of the camshaft, surrounded by a housing with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the rotor, and the chambers in the housing, as well. The housing""s outer circumference forms the sprocket, pulley or gear accepting drive, usually from the camshaft (typically a chain, belt or gears). The phaser operates using engine oil as the working fluid, introduced into the oil chambers on either side of vanes, so as to rotate the camshaft angularly relative to the drive from the crankshaft.
Since phasers cannot be perfectly sealed they are subject to the introduction of air into the system. When air is present in phaser it can cause rattling of the vane, an inability to hold phase angle, and an overall sluggish response. In the prior art, air that is present in the system is compressed in the vane chamber by torque reversals or is allowed to leak out through seals. The prior art does not provide an effective, efficient way in which to remove air present in the phaser.
In a variable cam timing (VCT) system, the timing gear on the camshaft is replaced by a variable angle coupling known as a xe2x80x9cphaser,xe2x80x9d having a rotor connected to the camshaft and a housing connected to (or forming) the timing gear, which allows the camshaft to rotate independently of the timing gear, within angular limits, to change the relative timing of the camshaft and crankshaft. The term xe2x80x9cphaser,xe2x80x9d as used here, includes the housing and the rotor, and all of the parts to control the relative angular position of the housing and rotor, allowing the timing of the camshaft to be offset from the crankshaft. In any of the multiple-camshaft engines, it will be understood that there would be one phaser on each camshaft, as is known to the art.
There are three common types of phasers: Cam Torque Actuated (CTA), Oil Pressure Actuated (OPA), and Torsion or Torque Assist (TA). In a CTA phaser, the variable cam timing system uses torque reversals in the camshaft caused by the forces of opening and closing engine valves to move the vane. Control valves are present to allow fluid flow from chamber to chamber causing the vane to move, or to stop the flow of oil, locking the vane in position. The CTA phaser has oil input to make up for losses due to leakage but does not use engine oil pressure to move the phaser.
In OPA or TA phasers, the engine oil pressure is applied to one side of the vane or the other, in the retard or advance chamber, to move the vane. The TA phaser adds check valves either one in each supply line to each chamber or one in the engine oil supply line to the spool valve. The check valves block oil pressure pulses due to torque reversals from propagating back into the oil system, and stop the vane from moving backward due to torque reversals. Motion of the vane due to forward torque effects is permitted.
In all three phasers, OPA, CTA, and TA, a spool valve controls the oil that is allowed to enter and exit from the vane chambers. The spool controls the exit and entry of oil by the placement of its lands. The position of the spool is controlled by a force solenoid which may be mechanical, electrical, or variable, or a differential pressure control system (DPCS). The spool valve is influenced towards the force solenoid by a spring. The spool valve commonly is in a bore in the rotor.
A variable camshaft phase adjustment device (phaser) for an internal combustion engine having at least one camshaft. The phaser has a housing having an outer circumference for accepting a drive force, and a rotor connected to a camshaft coaxially located within the housing. The housing and the rotor are capable of rotation to shift the relative angular position of the camshaft and the crankshaft. The spool valve comprising a spool slidably mounted within a bore in the rotor. In the spool a chamber is present that has an input communicating with the bore the spool is mounted in, an output communicating with the outside, and an air flow restriction. Hydraulic fluid from the input communicating with the bore is prevented from communicating with the outside by the air flow restriction. The air flow restriction is either in the input communicating with the bore or the output communicating with the outside.